Image processing apparatus for correcting color information and method therefor

ABSTRACT

According to one embodiment, there is provided an image processing apparatus including: first circuitry and second circuitry. The first circuitry sets for first color information in a first color gamut a correction quantity of a brightness defined depending on a lightness and a chroma of the first color information on a basis of a difference between a target brightness and the brightness of the first color information. The second circuitry corrects at least one of the lightness and the chroma of the first color information such that the brightness of the first color information is corrected by the correction quantity to obtain corrected color information in the first color gamut.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2013-145829, filed Jul. 11, 2013; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate to an image processing apparatus anda method therefor.

BACKGROUND

There has been discussed a method for performing color conversion withtaking into account human visual feature when color information existingin a color gamut is displayed on a display device having a color gamutdifferent therefrom. For example, there has been known a method forperforming color conversion on a color of the wide color gamut such thathue and gradation property perceived by a human are constant with takinginto account the human visual feature in order to display the colorinformation having a wide color gamut on an image display device havinga narrow color gamut.

However, this method has a difficulty in that although the colorinformation displayed on the image display device having the narrowcolor gamut seems to be equivalent to the color information having thewide color gamut in the hue and the gradation property, the chroma maydecrease accompanying the brightness decreasing as compared with thecolor information having the wide color gamut.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an image display device according to a firstembodiment;

FIG. 2 is a flowchart of an operation according to the first embodiment;

FIG. 3 is an illustration of a correction method of lightness and chromaaccording to the first embodiment;

FIG. 4 is an illustration of a correction method of lightness and chromaaccording to the first embodiment;

FIG. 5 is a diagram of an image display device according to a secondembodiment;

FIG. 6 is a flowchart of an operation according to the secondembodiment;

FIG. 7 is a diagram of an image display device according to a thirdembodiment; and

FIG. 8 is a diagram that illustrates a hardware configuration of animage processing device according to an embodiment.

DETAILED DESCRIPTION

According to one embodiment, there is provided an image processingapparatus including: first circuitry and second circuitry.

The first circuitry sets for first color information in a first colorgamut a correction quantity of a brightness defined depending on alightness and a chroma of the first color information on a basis of adifference between a target brightness and the brightness of the firstcolor information.

The second circuitry corrects at least one of the lightness and thechroma of the first color information such that the brightness of thefirst color information is corrected by the correction quantity toobtain corrected color information in the first color gamut.

Below, a description is given of embodiments of the present inventionwith reference to the drawings.

First Embodiment

FIG. 1 is a block diagram of an image display device according to afirst embodiment.

This image display device includes an image processing apparatus 100 anda display device 120 according to the embodiment. The image processingapparatus 100 includes a color gamut information acquisitor 102, aconversion unit 104, a setting unit 106, and a correction unit 108.

The color gamut information acquisitor 102 acquires color gamutinformation 103 a and 103 b from the outside or holds in the insidethereof in advance, and sends the color gamut information 103 a and 103b to the conversion unit 104. The color gamut information acquisitor 102may read the color gamut information 103 a and 103 b from a storage notshown or acquire the color gamut information 103 a and 103 b via a userinterface. The color gamut information 103 a is color gamut informationheld by an input image signal 101 to be input to the image processingapparatus 100. The color gamut information 103 b is color gamutinformation for the display device 120 which displays an image. A colorgamut of the display device 120 (first color gamut) is narrower than acolor gamut of an input image (second color gamut). For example, thecolor gamut of the display device 120 is encompassed by the color gamutof the input image. However, not all the color gamut of the displaydevice 120 may be encompassed by the color gamut of the input image, buta part thereof may be encompassed in some cases.

The conversion unit 104 performs color conversion on the input imagesignal 101 on the basis of the color gamut information 103 a to acquirea converted signal 105 a representing lightness, chroma, and hue, andsends the converted signal 105 a to the setting unit 106. The conversionunit 104 performs the color conversion on the input image signal 101 onthe basis of the color gamut information 103 b to acquire a convertedsignal 105 b representing lightness, chroma, and hue, and sends theconverted signal 105 b to the setting unit 106.

The setting unit 106 calculates a correction quantity of brightness inthe converted signal 105 b, more specifically, a correction quantity 107of at least one of the lightness and the chroma, and sends thecalculated correction quantity 107 to the correction unit 108.

The correction unit 108 corrects the converted signal 105 b inaccordance with the correction quantity 107 to obtain a corrected imagesignal 109. The correction unit 108 outputs the corrected image signal109 to the display device 120.

The display device 120 displays the corrected image signal 109 inputfrom the correction unit 108. In the embodiment, a case where thedisplay device 120 is a liquid crystal display is described as anexample. However, the display device 120 may be a plasma display or CRTdisplay, and may be a projection type device such as a projector.

Next, a description is given of an operation of the image processingapparatus 100 in the embodiment.

FIG. 2 is a flowchart showing the operation of the image processingapparatus 100 in the embodiment.

First, the color gamut information acquisitor 102 acquires the colorgamut information 103 a of the input image signal 101 and the colorgamut information 103 b of the display device 120 and sends these to theconversion unit 104 (S201).

The embodiment assumes that the color gamut of the display device 120 isa color gamut defined by ITU-R BT.709 which is a common color gamut fora display device such as a LCD. On the other hand, assumed is that thecolor gamut of the input image signal is a color gamut defined by ITU-RBT.2020 which is wider than the color gamut of the display device 120.These exemplary color gamuts are included in an example of a case wherethe color gamut of the input image signal 101 is wider than that of thedisplay device 120. The color gamuts of the display device 120 and theinput image 101 are not limited to ITU-R BT.709 and ITU-R BT.2020.

The conversion unit 104 converts the input image signal 101 into theconverted signals 105 a and 105 b representing the lightness, chroma,and hue (S202).

Specifically, the conversion unit 104 first performs gamma conversion ofFormula 1 on a gradation value for each of R, G, and B subpixels of eachpixel of the input image signal 101 input in an RGB format.

$\begin{matrix}{{R_{in} = \left( \frac{R_{in}^{\prime}}{255} \right)^{\gamma}}{G_{in} = \left( \frac{G_{in}^{\prime}}{255} \right)^{\gamma}}{B_{in} = \left( \frac{B_{in}^{\prime}}{255} \right)^{\gamma}}} & (1)\end{matrix}$Where, R_(in)′, G_(in)′, and B_(in)′ are the gradation values of the R,G, and B subpixels respectively in an input video signal, and thegradation value is represented by 8-bits (0 to 255). R_(in), G_(in), andB_(in) are the gradation values obtained by performing the gammaconversion on R_(in)′, G_(in)′, and B_(in)′, and represented usingrelative values from 0 to 1. “γ” represents a gamma coefficient.

Here, a configuration for performing the gamma conversion by Formula 1is shown. However, as another method, a look-up table is prepared inwhich an input gradation value and a gradation value after the gammaconversion are associated with each other in advance and a gammaconversion operation may be performed by referring the look-up table.The above gamma conversion is performed on the values of the R, G, and Bsubpixels respectively of all pixels in the input video signal.

The conversion unit 104 receives from the color gamut informationacquisitor 102 the color gamut information 103 a of BT.2020 as the colorgamut of the input image and the color gamut information 103 b of BT.709as the color gamut of the display device.

The conversion unit 104 calculates, from the color gamut information 103a and the input image signal 101, tristimulus values (X₇₀₉, Y₇₀₉, Z₇₀₉)converted on the basis of the color gamut defined by BT.709. Also,calculated, from the color gamut information 103 b and the input imagesignal 101, are tristimulus values (X₂₀₂₀, Y₂₀₂₀, Z₂₀₂₀) converted onthe basis of the color gamut defined by BT.2020.

A calculation formula for the tristimulus values (X₇₀₉, Y₇₀₉, Z₇₀₉) isrepresented by Formula 2A, and a calculation formula for the tristimulusvalues (X₂₀₂₀, Y₂₀₂₀, Z₂₀₂₀) is represented by Formula 2B.

$\begin{matrix}{\begin{bmatrix}X_{709} \\Y_{709} \\Z_{709}\end{bmatrix} = {M\begin{bmatrix}R_{in} \\G_{in} \\B_{in}\end{bmatrix}}} & \left( {2A} \right) \\{\begin{bmatrix}X_{2020} \\Y_{2020} \\Z_{2020}\end{bmatrix} = {N\begin{bmatrix}R_{in} \\G_{in} \\B_{in}\end{bmatrix}}} & \left( {2B} \right)\end{matrix}$Where, “M” is a color converting matrix of 3×3 representing the colorgamut information 103 a, which is the converting matrix for conversioncorrespondingly to a maximum color reproducing area reproduced by acolor gamut BT.709. “N” is a color converting matrix of 3×3 representingthe color gamut information 130 b, which is the converting matrix forconversion correspondingly to a maximum color reproducing areareproduced by a color gamut BT.2020.

Here, the color converting matrix is held for calculating thetristimulus values, which are calculated for each pixel from R_(in),G_(in), and B_(in). As another method, a relationship between thetristimulus values obtained by the color conversion, and R_(in)′,G_(in)′, and B_(in)′ is held in the look-up table, and the tristimulusvalues may be found for each pixel from R_(in)′, G_(in)′, and B_(in)′ byreferring the look-up table.

In a case where the input image signal is input in a format other thanthe RGB such as YUV, the XYZ tristimulus values may be found byreferring the LUT for converting into the XYZ tristimulus valuesdirectly from an input signal value of the YUV or the like.

Next, the tristimulus values X₇₀₉, Y₇₀₉ and, Z₇₀₉ calculated by thecolor conversion are converted into L*₇₀₉, a*₇₀₉, and b*₇₀₉ of a CIEL*a*b* color space. L*₇₀₉, a*₇₀₉, and b*₇₀₉ are calculated in accordancewith Formula 3.

$\begin{matrix}{{L_{709}^{*} = {{116 \times {f\left( \frac{Y_{709}}{Yw} \right)}} - 16}}{a_{709}^{*} = {500 \times \left\{ {{f\left( \frac{X_{709}}{Xw} \right)} - {f\left( \frac{Y_{709}}{Yw} \right)}} \right\}}}{b_{709}^{*} = {200 \times \left\{ {{f\left( \frac{Y_{709}}{Yw} \right)} - {f\left( \frac{Z_{709}}{Zw} \right)}} \right\}}}} & (3)\end{matrix}$Here, f(Y₇₀₉/Y_(w)) is calculated as shown in Formula 4, andf(X₇₀₉/X_(w)) and f(Z₇₀₉/Z_(w)) are also calculated similarly.

$\begin{matrix}{{{f\left( \frac{Y_{709}}{Yw} \right)} = {{7.787 \times \left( \frac{Y_{709}}{Yw} \right)} + {\frac{16}{116}\left( {\frac{Y_{709}}{Yw} \leq 0.008856} \right)}}}{{f\left( \frac{Y_{709}}{Yw} \right)} = \left( \frac{Y_{709}}{Yw} \right)^{\frac{1}{3}}}} & (4)\end{matrix}$X_(w), Y_(w), and Z_(w) represent the tristimulus values of a perfectreflecting diffuser. Further, a*₇₀₉ and b*₇₀₉ are converted by Formula 5into a chroma C*₇₀₉ and a hue h₇₀₉.

$\begin{matrix}{{C_{709}^{*} = \left\{ {\left( a_{709}^{*} \right)^{2} + \left( b_{709}^{*} \right)^{2}} \right\}^{\frac{1}{2}}}{h_{709} = {\tan^{- 1}\left( \frac{b_{709}^{*}}{a_{709}^{*}} \right)}}} & (5)\end{matrix}$

Similar to Formula 3 to Formula 5, a lightness L*₂₀₂₀, a chroma C*₂₀₂₀,and a hue h₂₀₂₀ are also calculated from the tristimulus values X₂₀₂₀,Y₂₀₂₀ and, Z₂₀₂₀.

The input image color information (lightness L*₂₀₂₀, chroma C*₂₀₂₀, andhue h₂₀₂₀) calculated in this way are sent as the converted signal 105 ato the setting unit 106 and the correction unit 108. The display devicecolor information (lightness L*₇₀₉, chroma C*₇₀₉, and hue h₇₀₉) are sentas the converted signal 105 b to the setting unit 106 and the correctionunit 108.

Next, the setting unit 106 calculates the correction quantity 107 of thelightness and chroma in the converted signal 105 b from the convertedsignals 105 a and 105 b (S203).

Specifically, first, the setting unit 106 calculates brightnessperceived respectively for the input image color information (L*₂₀₂₀,C*₂₀₂₀, and h₂₀₂₀) and the display device color information (L*₇₀₉,C*₇₀₉, and h₇₀₉).

Here, the perceived brightness is described. As is known as theHelmholtz-Kohlrausch effect, human eyes generally perceive that achromatic color is brighter than an achromatic color if the lightness isthe same, and perceive that the more vivid, the brighter.

The embodiment assumes that, on the basis of the Helmholtz-Kohlrauscheffect, a brightness B* perceived by the human eyes is defined dependingon a lightness L*, chroma C*, and hue h of a target object in accordancewith Formula 6.B*=L*+(F(h)+T)×C*  (6)However, “F” is a function for outputting values different depending onthe hue, and “T” is a constant. In accordance with Formula 6, abrightness B*₂₀₂₀ with which the color information of the input image isperceived is calculated as Formula 7A. Similarly, in accordance withFormula 6, a brightness B*₇₀₉ with which the color information in thedisplay device 120 is perceived is calculated as Formula 7B.B* ₂₀₂₀(L* ₂₀₂₀ ,C* ₂₀₂₀ ,h ₂₀₂₀)=L* ₂₀₂₀+(F(h ₂₀₂₀)+T)×C* ₂₀₂₀  (7A)B* ₇₀₉(L* ₇₀₉ ,C* ₇₀₉ ,h ₇₀₉)=L* ₇₀₉+(F(h ₇₀₉)+T)×C* ₇₀₉  (7B)

Further, a difference ΔB* between both perceived brightnesses iscalculated as the correction quantity of the brightness. The differenceΔB* between the brightness B*₂₀₂₀ (L*₂₀₂₀, C*₂₀₂₀, h₂₀₂₀) with which thecolor information of the input image is perceived and the brightnessB*₇₀₉ (L*₇₀₉, C*₇₀₉, h₇₀₉) with which the color information of thedisplay device 120 is perceived is calculated as Formula 8.

$\begin{matrix}\begin{matrix}{{\Delta\; B^{*}} = {{B_{2020}^{*}\left( {L_{2020}^{*},C_{2020}^{*},h_{2020}} \right)} -}} \\{B_{709}^{*}\left( {L_{709}^{*},C_{709}^{*},h_{709}} \right)} \\{= {\left( {L_{2020}^{*} - L_{709}^{*}} \right) + {\left( {{F\left( h_{2020} \right)} + T} \right) \times}}} \\{C_{2020}^{*} - {\left( {{F\left( h_{709} \right)} + T} \right) \times C_{709}^{*}}}\end{matrix} & (8)\end{matrix}$

Next, from the difference ΔB* between the perceived brightnesses,correction quantities ΔL* and ΔC* of the lightness and chroma are setwith respect to the color information (L*₇₀₉, C*₇₀₉, h₇₀₉) in thedisplay device 120.

For example, if the difference ΔB* (correction quantity of thebrightness) between the perceived brightnesses is entirely correctedusing the lightness, the correction quantity ΔL* of the lightness andthe correction quantity ΔC* of the chroma are calculated as Formula 9 inconsideration of Formula 6.ΔL*=ΔB*ΔC*=0  (9)

On the other hand, if the difference ΔB* between the perceivedbrightnesses is entirely corrected using the chroma, the correctionquantity ΔL* of the lightness and the correction quantity ΔC* of thechroma are calculated as Formula 10 in consideration of Formula 6.

$\begin{matrix}{{{\Delta\; L^{*}} = 0}{{\Delta\; C^{*}} = \frac{\Delta\; B^{*}}{\left( {{F\left( h_{709} \right)} + T} \right)}}} & (10)\end{matrix}$

Color information (L*_(L), C*_(L), h_(L)) having the difference ΔB*between the perceived brightnesses with only the lightness beingcorrected is calculated as Formula 11A. Color information (L*_(C),C*_(C), h_(C)) having the difference ΔB* between the perceivedbrightnesses with only the chroma being corrected is calculated asFormula 11B.

$\begin{matrix}{{L_{L}^{*} = {L_{709}^{*} + {\Delta\; B^{*}}}}{C_{L}^{*} = C_{709}^{*}}{h_{L} = h_{709}}} & \left( {11A} \right) \\{{L_{C}^{*} = L_{709}^{*}}{C_{C}^{*} = {C_{709}^{*} + \frac{\Delta\; B^{*}}{\left( {{F\left( h_{709} \right)} + T} \right)}}}{h_{C} = h_{709}}} & \left( {11B} \right)\end{matrix}$

Here, FIG. 3 shows, in the color reproducing area represented by thelightness, chroma, and hue, the color information (L*₇₀₉, C*₇₀₉, h₇₀₉)in the display device 120, the color information (L*_(L), C*_(L), h_(L))with only the lightness being corrected, and the color information(L*_(C), C*_(C), h_(C)) with only the chroma being corrected. Ahorizontal axis indicates the chroma and a vertical axis indicates thelightness. A solid line represents a color gamut boundary of the displaydevice, and a broken line represents a color gamut boundary of the inputimage. The color gamut of the input image is wider than the color gamutof the display device.

In FIG. 3, a line connecting the color information (L*_(L), C*_(L),h_(L)) with the color information (L*_(C), C*_(C), h_(C)) corresponds toa graph representing a relationship between the lightness and the chromawhere the perceived brightness B* represented by Formula 6 is constant.The color information existing on this line has the constant perceivedbrightness B* represented by Formula 6. For this reason, the colorinformation existing on this line is the same as the color information(L*₂₀₂₀, C*₂₀₂₀, h₂₀₂₀) of the input image in the perceived brightness.

Accordingly, the setting unit 106 sets a correction quantity ΔL*_(out)of the lightness and a correction quantity ΔC*_(out) of the chroma suchthat the color information after the correction with respect to thecolor information (L*₇₀₉, C*₇₀₉, h₇₀₉) of the display device 120 ispositioned on the line connecting the color information (L*_(L), C*_(L),h_(L)) with the color information (L*_(C), C*_(C), h_(C)).

For example, the ΔL*_(out) and the ΔC*_(out) may be a correctionquantity for correcting only with the lightness as shown by Formula 9 ora correction quantity for correcting only with the chroma as shown byFormula 10.

A correction quantity for correcting both the lightness and the chromamay be set as shown by Formula 12 by dividing the difference ΔB* betweenthe perceived brightnesses for the lightness and the chroma inaccordance with a constant “α” (ratio given in advance). However, “α” isa constant having a value from 0 to 1. In other words, (L*₇₀₉, C*₇₀₉,h₇₀₉) is corrected to the color information corresponding to a pointwhich is obtained by internally dividing by the ratio “α” given inadvance a portion from point color information (L*_(L), C*_(L), h_(L))having the chroma value the same as (L*₇₀₉, C*₇₀₉, h₇₀₉) to a point(L*_(C), C*_(C), h_(C)) having the lightness value the same as (L*₇₀₉,C*₇₀₉, h₇₀₉).

$\begin{matrix}{{{\Delta\; L_{out}^{*}} = {\Delta\; B^{*} \times \alpha}}{{\Delta\; C_{out}^{*}} = \frac{\Delta\; B^{*} \times \left( {1 - \alpha} \right)}{\left( {{F\left( h_{709} \right)} + T} \right.}}} & (12)\end{matrix}$

The setting unit 106 sends the calculated ΔL*_(out) and ΔC*_(out) as thecorrection quantity 107 to the correction unit 108.

The correction unit 108 calculates the corrected image signal 109 fromthe converted signal 105 b and the correction quantity 107 (S204).

Specifically, corrected color information (L*_(out), C*_(out), h*_(out))is calculated from the correction quantity ΔL*_(out) of the lightness,the correction quantity ΔC*_(out) of the chroma, and the colorinformation (L*₇₀₉, C*₇₀₉, h₇₀₉) in accordance with Formula 13. Here,the ΔL*_(out) and the ΔC*_(out) satisfy Formula 6 for the difference ΔB*between the perceived brightnesses calculated in accordance with Formula8. Therefore, the brightness with which the color information (L*_(out),C*_(out), h*_(out)) is perceived is the same as the brightness withwhich the color information (L*₂₀₂₀, C*₂₀₂₀, h*₂₀₂₀) is perceived. FIG.3 shows a case where an intersection between the line connecting thecolor information (L*_(L), C*_(L), h_(L)) with the color information(L*_(C), C*_(C), h_(C)) and the color gamut boundary of the displaydevice is (L*_(out), C*_(out), h_(out)), but not limited thereto. Adescription is given of a way of calculating the relevant intersectionin a second embodiment.L* _(out) =L* ₇₀₉ +ΔL* _(out)C* _(out) =C* ₇₀₉ +ΔC* _(out)h _(out) =h ₇₀₉  (13)The correction unit 108 converts C*_(out) and h*_(out) into a*_(out) andb*_(out) in accordance with Formula (14).a* _(out) =C* _(out)×cos(h _(out))b* _(out) =C* _(out)×sin(h _(out))  (14)The correction unit 108 converts L*_(out), a*_(out), and b*_(out) intotristimulus values X_(out), Y_(out), and Z_(out) in accordance withFormula (15).

$\begin{matrix}{{Y_{out} = {{f\left( \frac{Y}{Y_{w}} \right)} \times \frac{1}{7.787} \times {Yw}}}{{f\left( \frac{Y}{Y_{w}} \right)} \leq 0.206893}{Y_{out} = {\left( {f\left( \frac{Y}{Y_{w}} \right)} \right)^{3} \times {Yw}}}{0.206893 < {f\left( \frac{Y}{Y_{w}} \right)}}} & (15)\end{matrix}$X_(out) and Z_(out) are also calculated similar to Y_(out). However,f(X/X_(w)), f(Y/Y_(w)), and f(Z/Z_(w)) are calculated as Formula (16).

$\begin{matrix}{{{f\left( \frac{X}{X_{w}} \right)} = {\frac{a^{*}}{500} + \frac{\left( {L^{*} + 16} \right)}{116}}}{{f\left( \frac{Y}{Y_{w}} \right)} = \frac{\left( {L^{*} + 16} \right)}{116}}{{f\left( \frac{Z}{Z_{w}} \right)} = {\frac{\left( {L^{*} + 16} \right)}{116} - \frac{b^{*}}{200}}}} & (16)\end{matrix}$

The correction unit 108 converts X_(out), Y_(out), and Z_(out) intooutput signals R_(out), G_(out), and B_(out) in the RGB format inaccordance with the color reproducing area of the display device 120 asshown by Formula (17).

$\begin{matrix}{\begin{bmatrix}R_{out} \\G_{out} \\B_{out}\end{bmatrix} = {M^{- 1}\begin{bmatrix}X_{out} \\Y_{out} \\Z_{out}\end{bmatrix}}} & (17)\end{matrix}$However, “M⁻¹” is an inverse matrix of the color converting matrix M of3×3 shown by Formula (2).

Here, a description is given of a correction method in a case where theRGB signal values R_(out), G_(out), and B_(out) do not fall within asignal value range 0 to 1.

First, a minimum value of the RGB signal values R_(out), G_(out), andB_(out) is represented as L_(min)=min(R_(out), G_(out), B_(out)), and anoriginal value of L_(min) is represented as M_(min). In other words,M_(min) is any value of R_(in), G_(in), and B_(in). Here, if L_(min)falls below 0, values of the RGB signal values R_(out), G_(out), andB_(out) are updated in accordance with Formula (18).

Next, a maximum value of the updated RGB signal values R_(out), G_(out),and B_(out) is represented as L_(max)=max(R_(out), G_(out), B_(out)),and an original value of L_(max) is represented as M_(max). In otherwords, M_(max) is any value of R_(in), G_(in), and B_(in). Here, ifL_(max) exceeds 1, values of the RGB signal values R_(out), G_(out), andB_(out) are updated in accordance with Formula (19).

Finally, the RGB signal values (R_(out), G_(out), B_(out)) are output asthe corrected image signal 109.

$\begin{matrix}\begin{matrix}{R_{out} = {{\frac{\left( {0 - M_{m\; i\; n}} \right)}{\left( {L_{m\; i\; n} - M_{m\; i\; n}} \right)} \times \left( {R_{out} - R_{i\; n}} \right)} + R_{i\; n}}} \\{G_{out} = {{\frac{\left( {0 - M_{m\; i\; n}} \right)}{\left( {L_{m\; i\; n} - M_{m\; i\; n}} \right)} \times \left( {G_{out} - G_{i\; n}} \right)} + G_{i\; n}}} \\{B_{out} = {{\frac{\left( {0 - M_{m\; i\; n}} \right)}{\left( {L_{m\; i\; n} - M_{m\; i\; n}} \right)} \times \left( {B_{out} - B_{i\; n}} \right)} + B_{i\; n}}}\end{matrix} & (18) \\\begin{matrix}{R_{out} = {{\frac{\left( {1 - M_{m\;{ax}}} \right)}{\left( {L_{m\;{ax}} - M_{m\;{ax}}} \right)} \times \left( {R_{out} - R_{i\; n}} \right)} + R_{i\; n}}} \\{G_{out} = {{\frac{\left( {1 - M_{m\;{ax}}} \right)}{\left( {L_{m\;{ax}} - M_{m\;{ax}}} \right)} \times \left( {G_{out} - G_{i\; n}} \right)} + G_{i\; n}}} \\{B_{out} = {{\frac{\left( {1 - M_{m\;{ax}}} \right)}{\left( {L_{m\;{ax}} - M_{m\;{ax}}} \right)} \times \left( {B_{out} - B_{i\; n}} \right)} + B_{i\; n}}}\end{matrix} & (19)\end{matrix}$

This correction allows that even if the RGB signal values fall outside arange of 0 to 1 and the represented color is outside the color gamut ofthe display device, correction can be made to a color on an intersectionbetween a line connecting an original color with a color obtained bycorrecting the perceived brightness and the color gamut boundary of thedisplay device.

Here, the “color obtained by correcting the perceived brightness” is acolor whose lightness and chroma are corrected and which exists outsidethe color gamut of the display device, and corresponds to a point P1shown in FIG. 4, for example. A process for fitting the point P1 in thecolor gamut of the display device is the correction in accordance withFormulas 18 and 19, and the point P1 is corrected into a point P2 as aresult or the correction of Formulas 18 and 19. In other words, the“color on an intersection between a line connecting an original colorwith a color obtained by correcting the perceived brightness and thecolor gamut boundary of the display device” is the point P2 which is anintersection between a line connecting (L*₇₀₉, C*₇₀₉) with the point P1and the color gamut boundary of the display device. This allows that apoint whose lightness and chroma are corrected and which is made to falloutside the color gamut can be plotted on an outline of the color gamutof the display device with the gradation property being maintained. Thatis, when the point internally divided by the “α” described above ispositioned outside the color gamut of the display device, correction ismade to the color information corresponding to an intersection between aline connecting the internally divided point with (L*₇₀₉, C*₇₀₉, h₇₀₉)and the color gamut boundary of the display device.

In the example shown with Formula 12, (L*₇₀₉, C*₇₀₉, h₇₀₉) is correctedto color information corresponding to the point which is obtained byinternally dividing a line connecting the color information (L*_(L),C*_(L), h_(L)) and (L*_(C), C*_(C), h_(C)) by the ratio “α” given inadvance, but another method may be used as below. That is, a portion, ofa line connecting the color information (L*_(L), C*_(L), h_(L)) and(L*_(C), C*_(C), h_(C)), included in the color gamut of the displaydevice is identified, and (L*₇₀₉, C*₇₀₉, h₇₀₉) is corrected to the colorinformation corresponding to a point obtained by internally dividing theidentified portion by a ratio given in advance. In this case, the pointof the color information after the correction exists in the color gamutof the display device, and thus the process in accordance with Formula18 and Formula 19 is not necessary.

As described above, according to the embodiment, the display devicehaving the narrow color gamut can also display the color having theperceived brightness the same as the color having the wide color gamut.

Second Embodiment

A description is given of a second embodiment. The embodiment has ageneral configuration the same as the first embodiment, and thus adifference from the first embodiment is described.

FIG. 5 shows a configuration diagram of an image display deviceaccording to the embodiment. Components the same as those in FIG. 1 aredenoted by the same reference signs. Differently from FIG. 1, a path isadded for feedback from the correction unit 108 to the setting unit 106.

In the first embodiment, the setting unit 106 determines at a time thecorrection quantity ΔL*_(out) of the lightness and the correctionquantity ΔC*_(out) of the chroma on the basis of the difference ΔB*between the perceived brightnesses in accordance with Formula 9 orFormula 10 or Formula 12.

On the other hand, in this embodiment, a process is repeated in whichthe correction unit 108 returns the corrected image signal calculatedfrom the correction quantity 107 set by the setting unit 106 to thesetting unit 106 and the setting unit 106 resets the correction quantityin accordance with the value of the corrected image signal 109. Thisallows as shown in FIG. 3 the correction quantity ΔL*_(out) of thelightness and the correction quantity ΔC*_(out) of the chroma to becalculated which may correct the color information (L*₇₀₉, C*₇₀₉, h₇₀₉)to the color information existing in the color gamut of the displaydevice 120 and having the maximum chroma, that is, the color informationof an intersection between the line connecting the color information(L*_(L), C*_(L), h_(L)) with the color information (L*_(C), C*_(C),h_(C)) and the color gamut of the boundary of the display device. Inthis way, the chroma is corrected in priority to the lightness in orderthat the chroma decrease in the color gamut of the input image isrestrained as much as possible while the perceived brightness is madethe same as in the color gamut of the input image signal. A case mayoccur where the chroma after the correction is higher than the chroma inthe color gamut of the input image signal, but there is thought to be nodifficulty so long as the perceived brightnesses are the same.

The embodiment uses a binary search method as a method for calculatingthe lightness correction quantity ΔL*_(out) and the chroma correctionquantity ΔC*_(out) such that the corrected color information exists inthe color gamut of the display device 120 and the chroma is maximum.However, an algorithm like this for calculating the correction quantityis not limited thereto, and other search algorithm may be used.

The process for calculating by the conversion unit 102 the convertedsignals 105 a and 105 b from the input image signal 101 and the colorgamut information 103 a and 103 b is the same as that in the firstembodiment and the description thereof is omitted.

In the following, a description is given of operations of the settingunit 106 and the correction unit 108. FIG. 6 shows a flowchart of thesecond embodiment.

First, the setting unit 106 calculates the difference ΔB* between theperceived brightnesses from the converted signal 105 in accordance withFormula 8, and calculates correction quantity initial values ΔL*(0) andΔC*(0) of the lightness and chroma respectively from ΔB* as shown inFormula 20 to send to the correction unit 108 (S501). The embodimentsets the correction quantities of the lightness and chroma as theinitial values ΔL*(0) and ΔC*(0) respectively such that the differenceΔB* between the perceived brightnesses is entirely corrected using thechroma. The value of ΔL*(0) is zero, of course.

$\begin{matrix}{{{\Delta\;{C^{*}(0)}} = \frac{\Delta\; B^{*}}{\left( {{F\left( h_{709} \right)} + T} \right)}}{{\Delta\;{L^{*}(0)}} = {{\Delta\; B^{*}} - {\Delta\;{C^{*}(0)} \times \left( {{F\left( h_{709} \right)} + T} \right)}}}} & (20)\end{matrix}$

The correction unit 108 corrects the color information using ΔL*(0),ΔC*(0), and a constant β as Formula 21 to calculate the lightness L*(0),chroma C*(0), and hue h(0). However, β is a constant having a valueranging from 0 to 1.C*(0)=C* ₇₀₉ +ΔC*(0)×βL*(0)=L* ₇₀₉ ΔL*(0)+{ΔC*(0)×(1−β)}×(F(h ₇₀₉)+T)h(0)=h ₇₀₉  (21)

Next, similarly to Formulas 14 to 17, the RGB signal values R(0), G(0),and B(0) in displaying the corrected color information (L*(0), C*(0),h(0)) on the display device are calculated from the lightness L*(0),chroma C*(0), and hue h(0) (S502).

Here, whether or not R(0), G(0), and B(0) all fall within a range from 0to 1 is determined (S503).

In a case where R(0), G(0), and B(0) all fall within a range from 0 to1, the correction unit 108 outputs R(0), G(0), and B(0) as the correctedimage signal 109 to the display device 120 (S504).

On the other hand, in a case where any of R(0), G(0), and B(0) does notfall within a range from 0 to 1, the correction unit 108 sends R(0),G(0), and B(0) to the setting unit 106. The setting unit 106 calculatesthe correction quantities ΔL*(1) and ΔC*(1) of the lightness and chromarespectively as Formula 22 to send to the correction unit 108 (S505).

$\begin{matrix}{{{\Delta\;{C^{*}(1)}} = \frac{\Delta\; B^{*}}{2 \times \left( {{F\left( h_{709} \right)} + T} \right)}}{{\Delta\;{L^{*}(1)}} = \frac{\Delta\; B^{*}}{2}}} & (22)\end{matrix}$

The correction unit 108, similarly to Formula 21, calculates thelightness L*(1), chroma C*(1), and hue h(1) to calculate, similarly toFormulas 14 to 17, the RGB signal values R(1), G(1), and B(1) indisplaying the color information (L*(1), C*(1), h(1)) on the displaydevice 120 (S506).

Here, assuming that the RGB signal values are represented as R(k), G(k),and B(k) in a generalized form using a number k of updates, whether ornot R(k), G(k), and B(k) all fall within a range from 0 to 1 isdetermined (S507).

Depending on the values of R(k), G(k), and B(k), the correctionquantities of the lightness and chroma are updated to ΔL*(k+1) andΔC*(k+1) respectively as shown by Formula 23 and Formula 24.

1. Case where R(k), G(k), and B(k) all fall within a range from 0 to 1(S508)

$\begin{matrix}{{{\Delta\;{C^{*}\left( {k + 1} \right)}} = {{\Delta\;{C^{*}(k)}} + \frac{\Delta\; B^{*}}{2^{k + 1} \times \left( {{F\left( h_{709} \right)} + T} \right)}}}{{\Delta\;{L^{*}\left( {k + 1} \right)}} = {{\Delta\; B^{*}} - {\Delta\;{C^{*}\left( {k + 1} \right)} \times \left( {{F\left( h_{709} \right)} + T} \right)}}}} & (23)\end{matrix}$2. Case where any of R(k), G(k), and B(k) does not fall within a rangefrom 0 to 1 (S509)

$\begin{matrix}{{{\Delta\;{C^{*}\left( {k + 1} \right)}} = {{\Delta\;{C^{*}(k)}} - \frac{\Delta\; B^{*}}{2^{k + 1} \times \left( {{F\left( h_{709} \right)} + T} \right)}}}{{\Delta\;{L^{*}\left( {k + 1} \right)}} = {{\Delta\; B^{*}} - {\Delta\;{C^{*}\left( {k + 1} \right)} \times \left( {{F\left( h_{709} \right)} + T} \right)}}}} & (24)\end{matrix}$Further, depending on the calculated correction quantities ΔL*(k+1) andΔC*(k+1), and the constant β, the lightness L*(k+1), chroma C*(k+1), andhue h(k+1) are updated as Formula 25.C*(k+1)=C* ₇₀₉ +ΔC*(k+1)×βL*(k+1)=L* ₇₀₉ +ΔL*(k+1)+{ΔC*(k+1)×(1−β)}×(F(h ₇₀₉)+T)h(k+1)=h(k)   (25)

Similarly to Formulas 14 to 17, the RGB signal values R(k+1), G(k+1),and B(k+1) in displaying the color information (L*(k+1), C*(k+1),h(k+1)) on the display device 120 are calculated (S510).

Here, values of L_(Diff) and C_(Diff) as the difference values of thelightness and chroma respectively are calculated as shown by Formula 26to check a magnitude relationship between L_(Diff) and a predefinedthreshold L_(TH) of the lightness, and a magnitude relationship betweenC_(Diff) and a predefined threshold C_(TH) of the chroma (S511).L _(Diff) =L*(k+1)−L*(k)C _(Diff) =C*(k+1)−C*(k)  (26)

In a case where both conditions of L_(Diff)≦L_(TH) and C_(Diff)≦C_(TH)are met, R(k+1), G(k+1), and B(k+1) are output as the corrected imagesignal 109 to the display device 120 (S504).

In a case where any condition of L_(Diff)≦L_(TH) and C_(Diff)≦C_(TH) isnot met, the correction unit 108 sends R(k+1), G(k+1), and B(k+1) to thesetting unit 106, and the setting unit 106 further updates the lightnessand chroma.

In the subsequent processes, steps S507 to S511 are repeated. At stepS511, in the above described embodiment, the difference of the lightnessand the difference of the chroma are compared with the thresholdsthereof respectively to determine the conditions. As another method, atstep S511, the processes may be repeated until a condition is met thatthe number k of updates reaches a predefined constant N.

If at step S511, the condition is determined to be met, the correctionunit 108 outputs the finally calculated RGB signal values R(k), G(k),and B(k) as the corrected image signal 109.

As described above, according to the embodiment, when the colorinformation is corrected, which is obtained by converting the inputimage signal having the wide color gamut held by the input image signalinto the color gamut of the display device, such that the colorinformation exists in the color gamut of the display device 120 and thebrightness the same as the color of the wide color gamut is to beperceived, the correction can be performed so as to make the chromalarge as much as possible.

Third Embodiment

A description is given of a third embodiment. The embodiment has ageneral configuration the same as the first embodiment, and thus adifference from the first embodiment is described.

FIG. 7 shows a configuration diagram of an image display deviceaccording to the embodiment. Components the same as those in FIG. 1 aredenoted by the same reference signs and duplicated description isomitted except for extended processes.

In the first embodiment, the color gamut of the display device 120 is acolor gamut defined by BT.709 and the color gamut of the input imagesignal is a color gamut defined by BT.2020, and the color gamut of thedisplay device 120 is far smaller than the color gamut of the inputimage signal. In this embodiment, a description is given of a case wherethe color gamut of the input image signal is also a color gamut definedby BT.709 similar to the display device 120.

In the embodiment, a look-up table (LUT) holding unit 110 is added. TheLUT holding unit 110 has a LUT 111. The LUT 111 is a table forconverting the input image signal having a narrow color gamut into thecolor information having a color gamut (third color gamut) wider thanthat.

The LUT 111 may be a LUT which is created estimating the colorinformation of an imaging target (e.g., sea's color, flower's color)with taking into account characteristics of an imaging apparatus imagingthe input image signal 101, or a LUT which is created estimating thecolor information of the imaging target with averagely taking intoaccount a general imaging apparatus in a case where the imagingapparatus is unknown like a TV broadcast wave. The LUT in this case isfor restoring a video to an original color gamut wider than that of theImaging apparatus, the video being generated in a manner that an imagingtarget originally having had a wider color gamut is compressed into anarrower color gamut as a result from imaging. The LUT 111 is notlimited to those, and may be any so long as the color information isobtained by converting the color gamut of the input image to anothercolor gamut.

The conversion of the color gamut may be done using not the LUT formatbut a function for nonlinear conversion.

The conversion unit 104 acquires the color gamut information 103 b ofthe display device from the color gamut information acquisitor 102 toconvert the input image signal similarly to Formula 1 to Formula 5 andcalculate the color information (L*₇₀₉, C*₇₀₉, h₇₀₉).

The conversion unit 104 acquires the LUT 111 from the LUT holding unit110 for converting the color gamut of the input image signal.

The conversion unit 104 refers the LUT 111 with respect to the RGBsignal values (R_(in), G_(in), B_(in)) as signals after the gammaconversion of the input image signal to calculate the color information(L*_(LUT), C*_(LUT), h*_(LUT)) of the color gamut different from thecolor gamut of the input image.

The conversion unit 104 sends the color information (L*₇₀₉, C*₇₀₉, h₇₀₉)as the converted signal 105 b and the color information (L*_(LUT),C*_(LUT), h*_(LUT)) as the converted signal 105 c to the setting unit106 and the conversion unit 108.

The setting unit 106 and the conversion unit 108 substitute the colorinformation (L*_(LUT), C*_(LUT), h*_(LUT)) for the color information(L*₂₀₂₀, C*₂₀₂₀, h*₂₀₂₀) in the first and second embodiments to performthe processes similar to the first and second embodiments. Thiscalculates the correction quantity 107 of the lightness and chroma andthe corrected image signal 109 converted from the converted signal 105b.

As described above, according to the embodiment, the color informationobtained by converting the input image signal into the color gamut ofthe display device is corrected so as to be able to perceive thebrightness the same as the color information obtained by converting theinput image signal using the LUT, allowing a user to perceive thebrightness the same as the color gamut of the LUT also in displaying onthe display device having the narrow color gamut.

FIG. 8 is a diagram that illustrates a hardware configuration of animage processing device according to an embodiment of the presentinvention.

The image processing device of each embodiment can be realized by usinga general-purpose computer device as basic hardware as illustrated in,for example, FIG. 8. In this computer device 200, a processor 202 suchas a CPU, a memory 203 and an auxiliary storage 204 such as a hard diskare connected with a bus 201, and a storage medium 206 is furtherconnected via an external I/F 205. The external I/F 205 may connect toan image displaying device. Each processing block in the imageprocessing device can be realized by making the processor 202 mounted onthe above-mentioned computer device execute a program. Any combinationof these blocks may constitute circuitry. At this time, the imageprocessing device may be realized by installing the above-mentionedprogram in the memory 203 or the auxiliary storage 204 of the computerdevice beforehand, or may be realized by storing it in the storagemedium 206 such as a “CD-ROM” or distributing the above-mentionedprogram through a network and arbitrarily installing this program in thecomputer device. Moreover, each buffer in the image processing devicecan be realized by arbitrarily using the memory 203, the hard disk 204or the storage medium 206 such as a “CD-R”, a “CD-RW”, a “DVD-RAM” and a“DVD-R”, which are incorporated or attached to the above-mentionedcomputer device.

Furthermore, the image processing apparatus may include a CPU (CentralProcessing Unit), a ROM (Read Only Memory) and a RAM as one example ofcircuitry. In this case, each unit or each element in the imageprocessing apparatus as shown in FIG. 1, 5 or 7 can be controlled by aCPU's reading out into a RAM and executing a program which is stored ina storage or ROM.

Also, the above-stated hardware configuration is one example and a partor all of the image processing apparatus according to an embodiment canbe realized by an integrated circuit such as a LSI (Large ScaleIntegration) or an IC (Integrated Circuit) chip set as one example ofcircuitry. Each function block in the image processing apparatus can berealized by a processor, individually, or a part or all of the functionblocks can be integrated and realized by one processor. A means for theintegrating the part or all of the function blocks is not limited to theLSI and may be dedicated circuitry or a general-purpose processor.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

The invention claimed is:
 1. An image processing apparatus comprising: first circuitry configured to calculate a difference between a brightness of first color information expressed in a first color gamut of a first device and a brightness of the first color information expressed in a second color gamut of a second device, the first color information including a first lightness and a first chroma, and to calculate a first correction quantity corresponding to a correction quantity of the first lightness and a second correction quantity corresponding to a correction quantity of the first chroma so as to represent the difference based on a relationship formula involving lightness, chroma and brightness; and second circuitry configured to output second color information including a second lightness and a second chroma, the second lightness corresponding to corrected lightness of the first lightness by the first correction quantity and the second chroma corresponding to corrected chroma of the first chroma by the second correction quantity, wherein the first circuitry is configured to find a portion in which values of the lightness and the chroma are equal to or lamer than values of the lightness and the chroma of the first color information in a graph representing a relationship between the lightness and the chroma capable of representing a brightness obtained by correcting the brightness of the first color information expressed in the first color gamut by a quantity depending on the difference, and to calculate the first correction quantity and the second correction quantity such that the second lightness and the second chroma correspond to a point which internally divides the portion by a given ratio.
 2. The apparatus according to claim 1, wherein the first circuitry is configured to calculate the first correction quantity such that a value of the first chroma becomes larger.
 3. The apparatus according to claim 1, wherein the first circuitry is configured to, when the internal dividing point is positioned outside a boundary of the first color gamut, calculate the first correction quantity and the second correction quantity such that the second lightness and the second chroma correspond to an intersection between the boundary of the first color gamut and a line connecting between the internal dividing point and a point representing the first color information.
 4. The apparatus according to claim 1, wherein the first circuitry is configured to find a portion in which values of the lightness and the chroma are equal to or larger than values of the lightness and the chroma of the first color information in a graph representing a relationship between the lightness and the chroma capable of representing a brightness obtained by correcting the brightness of the first color information expressed in the first color gamut by a quantity depending on the difference and which is included in the first color gamut, and to calculate the first correction quantity and the second correction quantity such that the second lightness and the second chroma correspond to a point which internally divides the portion by a given ratio.
 5. The apparatus according to claim 1, wherein the first color information includes a first hue, the second color information includes a second hue, and the first circuitry is configured to calculate the first correction quantity and the second correction quantity such that the second hue is maintained to have same hue as that of the first hue.
 6. The apparatus according to claim 1, wherein the first color gamut is wider than the second color gamut, and the apparatus further comprises third circuitry configured to generate the first color information by performing color conversion on an input image signal on a basis of the first color gamut, and to generate second color information by performing color conversion on the input image signal on a basis of the second color gamut.
 7. The apparatus according to claim 1, wherein the apparatus further comprises third circuitry configured to generate the second color information by performing color conversion on an input image signal on a basis of the second color gamut, and to generate the first color information by performing color conversion on the input image signal on a basis of a third color gamut wider than the first and second color gamuts.
 8. The apparatus according to claim 1, wherein the relationship formula among lightness, chroma and brightness is: B*=L*+(F(h)+T)×C* wherein B* is brightness, L* is lightness, “F” is a function for outputting a value depending on a hue, “T” is a constant, and C* is chroma.
 9. An image processing method comprising: calculating a difference between a brightness of first color information expressed in a first color gamut of a first device and a brightness of the first color information expressed in a second color gamut of a second device, the first color information including a first lightness and a first chroma; calculating a first correction quantity corresponding to a correction quantity of the first lightness and a second correction quantity corresponding to a correction quantity of the first chroma so as to represent the difference based on a relationship formula involving lightness, chroma and brightness; outputting second color information including a second lightness and a second chroma, the second lightness corresponding to a corrected lightness of the first lightness by the first correction quantity and the second chroma corresponding to a corrected chroma of the first chroma by the second correction quantity, wherein the calculating of the first correction quantity and the second correction quantity comprises: finding a portion in which values of the lightness and the chroma are equal to or lamer than values of the lightness and the chroma of the first color information in a graph representing a relationship between the lightness and the chroma capable of representing a brightness obtained by correcting the brightness of the first color information expressed in the first color gamut by a quantity depending on the difference; and calculating the first correction quantity and the second correction quantity such that the second lightness and the second chroma correspond to a point which internally divides the portion by a given ratio.
 10. An image processing apparatus comprising: a processor; and a memory configured to store data based on a relationship formula involving lightness, chroma and brightness and to store processor-executable instructions that, when executed by the processor, cause the processor to: calculate a difference between a brightness of first color information expressed in a first color gamut of a first device and a brightness of the first color information expressed in a second color gamut of a second device, the first color information including a first lightness and a first chroma; calculate a first correction quantity corresponding to a correction quantity of the first lightness and a second correction quantity corresponding to a correction quantity of the first chroma so as to represent the difference based on the relationship; and output second color information including a second lightness and a second chroma, the second lightness corresponding to a corrected lightness of the first lightness by the first correction quantity and the second chroma corresponding to a corrected chroma of the first chroma by the second correction quantity, wherein the calculating of the first correction quantity and the second correction quantity comprises: finding a portion in which values of the lightness and the chroma are equal to or larger than values of the lightness and the chroma of the first color information in a graph representing a relationship between the lightness and the chroma capable of representing a brightness obtained by correcting the brightness of the first color information expressed in the first color gamut by a quantity depending on the difference; and calculating the first correction quantity and the second correction quantity such that the second lightness and the second chroma correspond to a point which internally divides the portion by a given ratio. 